Now we use the new Image-customization framework in Gluon, we need to
also update the docs so examples and descriptions are acurate again.

Signed-off-by: David Bauer <mail@david-bauer.net>

Signed-off-by: David Bauer <mail@david-bauer.net>

Signed-off-by: Stefan Weil <sw@weilnetz.de>

Signed-off-by: David Bauer <mail@david-bauer.net>

closes freifunk-gluon/gluon#2603

Co-authored-by: Martin Weinelt <martin@darmstadt.freifunk.net>

- Restructure page
- Add information on how to add L2TPv3 offloading support to a build
  using configurable ciphers. The null method is not reocmmended anymore.
- Add notes and pointers regarding the gateway configuration to provide
  gateway admins with hints on how to modify their configuration to
  accommodate this new feature.
- Mention wireguard support

Based-on-patch-by: Felix Kaechele <felix@kaechele.ca>

2 spaces is the most common indentation width used in the docs; adjust
the rest for consistency.

Also change .editorconfig accordingly.